Abstract

Convective removal of part of the thermal boundary layer and its replacement with hotter asthenosphere has been proposed to explain the unusual tectonic setting of the Tibetan plateau on Earth: a topographically highstanding plateau abutting an active region of continental collision and yet undergoing extensional tectonics. A similar model, adapted to take account of the probable absence of plate tectonic processes on Mars, can explain three of the distinctive morphological peculiarities of the shield volcano Alba Patera, namely, the extremely low angle of its flank slopes, the system of annular graben which surround the edifice, and the fact that the nature of volcanic activity apparently changed during the lifetime of the volcano from volatile rich to volatile poor. This change in eruptive style has previously been interpreted to be a consequence of the evolution of the Martian mantle, but we argue that the change is because late stage activity at Alba Patera is sourced from remelted underplated igneous material rather than juvenile melt from the mantle. Our model can also explain the tectonic geomorphology of the Syria Planum area and can relate the compressional regime which produced wrinkle ridges to the more abundante extensional tectonics. These two sets of features have previously been analyzed separately and taken as evidence for global heating or cooling, respectively. Our proposal removes the requirement to explain these features by diametrically opposed processes and on a global scale identifies the patera and large shield morphologies as the end-members of a single evolutionary sequence.